Heparan sulfate proteoglycans (HSPGs) are abundant molecules found on cell surfaces and in the extracellular matrix. In the last decade these molecules have been shown to play critical roles in controlling responses of cells to secreted growth factors, and the distributions of signaling molecules in the matrix. Our long-term objectives are to understand how these molecules affect cellular behavior and the assembly of tissues during development. Recent work in my lab using the model organism Drosophila has demonstrated roles for HSPGs in shaping the gradient of the Wingless (Wg) morphogen, controlling axon guidance in the visual system, and influencing the form and function of the neuromuscular synapse. The research we propose has three broad but related aims, 1) determine if HSPGs control the distribution of Wg by regulating endocytosis and intracellular trafficking, 2) establish if HSPGs affect synapse assembly by controlling endocytosis and growth factor signaling, and 3) determine if HSPGs govern responses of growth cones to guidance cues, or the distributions of guidance cues in the extracellular matrix of the brain. Using antibodies directed against Wg and markers for different endosomal compartments, we will assess the trafficking of Wg in mutants affecting a specific HSPG, Dally-like, and a secreted enzyme, Notum, that removes Dip from cell surfaces. We propose that regional control of Wg gradients are achieved by a combination of Dip and Notum activity in the developing wing. Our studies of the synapse employ direct measures of endocytosis using fluorescent dyes that are incorporated into endocytosed vesicles, visualization of growth factors critical for synapse development, and genetic studies to identify the signaling systems affected by HSPGs in the synapse. Finally, we will use genetic strategies to determine in what cell types HSPG must be expressed to permit normal axon guidance in the visual system, and what axon guidance systems require these molecules. HSPGs control signaling mediated by numerous growth factors that affect tumor development, angiogenesis, tissue repair, and chemokine responses in immune system function. Heparan sulfate biosynthesis is required for normal bone development and specific HSPGs affect growth regulation in humans. A detailed cellular and molecular understanding of these diverse molecules is therefore of fundamental importance in a number of fundamental processes affecting human disease.